1. Field of the Invention
This invention relates to a mass spectrometer incorporating an ion source adapted for the analysis of solutions, especially the eluent from a liquid chromatographic column, and to such a source. More particularly it relates to a thermospray type of liquid chromatograph-mass spectrometer interface in which the solvent is removed and the sample is ionized through the generation by thermal means of a spray containing electrically charged species.
2. Related Art
The use of a mass spectrometer to analyse the eluent from liquid chromatographic columns presents many difficulties, and a wide variety of methods have been proposed in the last few years. These have been reviewed by W. H. McFadden in Journal of Chromatographic Science, 1979, vol. 17, pps. 2-16, by P. J. Arpino and G. Guiochon in Analytical Chemistry, 1979, volume 51pps. 638A-701A, and by D. A. Yorke in Proc. Inst. Petrol. (London), 1982, vol. 2, pps 159-170. A major problem encountered when combining a liquid chromatograph with a mass spectrometer is that the eluent from a typical liquid chromatograph may consist of several ml/minute of a very dilute solution, frequently in an aqueous solvent, while most mass spectrometer ion sources operate in a high vacuum and can accept only a very small quantity of such a solution, typically not exceeding 0.1 ml/minute. In order to avoid excessive loss of sample, therefore, the interface between the chromatograph and the ion source must be capable of removing a large proportion of the solvent while transmitting the bulk of the sample. Further, liquid chromatography is often employed for the separation of thermally unstable compounds which cannot easily be ionized in a conventional ion source without decomposition or fragmentation. Although many techniques for the ionization of compounds of this type are now available, e.g., field desorption and neutral particle or ion bombardment secondary ion mass spectrometry, most of these methods require the sample to be coated on an emitter or target, and are consequently unsuited for the ionization of the eluent from a liquid chromatograph. It is highly desirable, therefore, that any method of interfacing a liquid chromatograph to a mass spectrometer should incorporate a method of ionizing samples which are thermally unstable or involatile. Several interfacing methods which fulfil these requirements are known, and the present invention is concerned with the method known as thermospray ionization, which has been developed chiefly by M. L. Vestal and his coworkers.
The first practical version of this interface an ion source is described in International patent application publication No. 81/03394. In the method, the eluent from the liquid chromatograph is passed through a capillary tube which is heated, typically to red heat, by means of four oxy-hydrogen torches, so that a jet of solvent vapour containing particles of sample is formed in a region of reduced pressure downstream of the end of the capillary tube. A conical skimmer containing a small hole in the centre is used to intercept the jet thus formed, and the vapour emerging through the hole (which contains a greater proportion of sample molecules) enters a tubular channel, also maintained at a reduced pressure, where it impinges on a heated target situated downstream of the skimmer. An ion vapour is supposedly formed at this point and ions characteristic of the dissolved sample leave the tubular chamber through a hole in its wall adjacent to the heated target and are mass analyzed by a mass filter situated in a region of high vacuum. Surprisingly, the resultant mass spectra show no evidence of thermal decomposition, even when thermally unstable samples are analyzed, and this is ascribed to the very short time of passage through the interface and to the protection of the sensitive molecules from exposure to the very high temperatures by virtue of the evaporating solvent molecules. In the patent application, emphasis is placed on the presence of the heated target, and the inventors claim that ionization of sample molecules takes place by an imperfectly understood process associated with the collision of the molecular beam of solvent molecules and solvated sample molecules with the heated surface of the target. Later work by C. R. Blakley and M. L. Vestal, described in Analytical Chemistry, 1983, vol. 55, p 750, showed that in fact the heated target could be omitted, and that ions were formed in the spray emerging from the heated capillary. The apparatus was also simplified by omission of the conical skimmer and the region of reduced pressure between it and the capillary nozzle. In this simplified version, the solution to be analysed is pumped through a strongly heated capillary tube which protrudes into a wider bore tube maintained at a pressure of between 1 and 10 torr by means of a mechanical vacuum pump. A spray of evaporating solvent droplets containing ions and molecules of the solvent is formed in the wider bore tube. A small hollow cone with a hole in its apex is positioned through the wall of the tube downstream of the end of the capillary with its axis at right angles to that of the spray and with its apex just short of the centre of the tube. The hole in the apex of the cone leads to the high vacuum part of the mass spectrometer, and ions passing through it enter the mass filter. The walls of the tube enclosing the spray and the sampling cone are advantageously heated.
The thermospray method described is quite distinct from the earlier method of connecting a liquid chromatograph and a mass spectrometer in which the eluent was directly introduced into the source of a conventional chemical ionization mass spectrometer, through a heated capillary, for example as described in U.S. Pat. No. 3,997,298. In ion sources of this kind, ionization of the sample is effected by reaction of the sample molecules with a plasma of ions formed from the solvent vapour by means of collisions with a beam of electrons inside the source. It is also distinct from liquid analyzing mass spectrometers based on techniques such as electrospray or electrohydrodynamic ionization, for example as described in U.S. Pat. Nos. 4,160,161, 4,209,696, and 4,144,451, in which ionization is effected by means of a high electrical potential applied between the capillary tube and a counter electrode situated opposite its end. In these systems a spray of charged droplets is generated as the liquid emerges from the capillary tube under the influence of the electrical field, and these droplets gradually evaporate leaving ions, usually clustered with solvent molecules, which are caused to pass through a hole in the counter electrode into a mass filter.
A possible explanation of the mechanism of thermospray ionization has been offered by M. L. Vestal in the International Journal of Mass Spectrometry and Ion Physics, 1983, vol. 46, pps. 193-6, where it is suggested that ions already present in the solution evaporate in solvated form from droplets in the thermally produced spray. For example, if the solution to be analyzed consists of the sample dissolved in a solution of ammonium acetate, primary ions of NH.sub.4.sup.+ and CH.sub.3 COO.sup.- are formed in the spray. These ions can react with sample molecules forming NH.sub.4.sup.+ or CH.sub.3 COO.sup.- adducts, which can be analysed by the mass spectrometer. In some cases, (M+H).sup.+ ions or (M-H).sup.- ions are formed, especially when the sample molecule M is relatively low molecular weight and neutral in solution, presumably by a mechanism similar to that of conventional chemical ionization. This mechanism implies that optimum conditions for thermospray ionization occur when the liquid chromatograph eluent contains ions such as NH.sub.4.sup.+ and CH.sub.3 COO.sup.-, and this is found to be the case. Fortunately, mobile phases containing salts are commonly employed in the liquid chromatography of biochemicals, and consequently the method is very useful in this field. It is found that the best results for high molecular weight thermally unstable compounds are obtained when the compounds are ionized in the solution of the eluent, and rather poorer results are obtained with high molecular weight samples which are electrically neutral in solution. Indeed, many molecules of this type cannot be successfully ionized by the method. Even when the method produces good results, the ions formed from high molecular weight compounds are usually clustered with a variety of species derived from the mobile phase, which increases the difficulty of spectrum interpretation. Further, all prior art forms of the thermospray method rely on the diffusion of sample ions through the small hole in the sampling cone, along an axis at right angles to the direction of the spray. This method of sampling reduces the number of neutral molecules entering the high vacuum part of the mass spectrometer, which leads to a simplification of the pumping system, but it is relatively inefficient because the majority of the ions formed will be carried past the sampling cone by the motion of the molecules in the spray.
It is an object of the present invention to provide a mass spectrometer having a thermospray type liquid ionization source which incorporates means for improving the efficiency of sampling the ions in the spray, and additionally to improve the efficiency of ionization, especially of samples of high molecular weight which are electrically neutral in solution, thereby increasing the sensitivity of the apparatus. It is a further object of the invention to provide a mass spectrometer having a thermospray type liquid ionization source in which the proportion of ions formed by the addition to or abstraction from the sample molecule of a proton is increased relative to that of the cluster ions which are predominantly formed with prior art thermospray type ion sources, and to increase in a controlled way the proportion of fragment ions formed by decomposition of the primary ions produced in the spray, so that interpretation of the resulting spectra may be facilitated and the determination of the molecular weight and the structure of the sample may be made more certain.